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In optical LIN converter PCB there is a weird element glued: A plastic ring with resin/ plastic fulfillment.

Is it some sort of local coating or chip-on-board encapsulation? There are no elements in this area on the opposite side of PCB, just 2 Vias.

enter image description here

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In the PCB industry this is called component potting.

The white ring is there to ensure the pouring of the resin can reach the required height and not spill on the nearby components. What component is under there is tricky to know but you can make educated guess:

  • it is not for weather proofing (the rest of the board would be coated too)
  • it is a component that is high enough to require a pouring "ring"
  • it is the only one with this resin contraption
  • it doesn't have a lot of pins
  • the resin is not translucent thus it is not an optoelectronic component

Thus:

  • it is not some kind of IC or flip chip protection (too many pins according to your observation and it wouldn't need the "ring")
  • it might be some kind of fragile inductor (sealed for protection of its winding)
  • it might be a trimming potentiometer (potted as a mean of sealing its calibration)

For me looking at the size of this I would tend to say it is a trimming potentiometer that was sealed after calibration, the white ring being there to ensure the resin was poured high enough. The resin making the calibration more permanent than anything else (that is maybe why they have gone with this instead of a dab of glue).

This might be entirely wrong, I am not the one who designed this.

EDIT/FOLLOW-UP:

From the extanded details of the later answers and comments ! It is clear that I was wrong... I think we can now confortably assume that the potting was done as a sound insulating feature to reduce audible noise from the SMPS (that was the ahah factor I think).

In rare cases it may be necessary to varnish/encase the entire powersupply in potting material to reduce audible noise.There are many companies that make these electronics insulation resins and polymers for these applications such as Elantas Electrical Insulation,EpicResins and ITW Engineered Polymers. (cited from http://www.ti.com/lit/an/slua821/slua821.pdf)

That was a very good question ! I definitely learned something new. And I wish more powersupply manufacturer would implement these.

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    \$\begingroup\$ "it is not for weather proofing (the rest of the board would be coated too)" - perhaps it is for 'weatherproofing' a particularly sensitive circuit on the board? \$\endgroup\$ – Bruce Abbott Sep 10 '19 at 21:54
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Found whats underneath in documentation.

Looks like SMPS circuit. So this "Component Potting" as @benguru mentioned might be used for either:

  • Better Thermal Dissipation
  • Reducing the sound noise on ceramic RC Circuit
  • Whiskers
  • Might be used as EMC shielding but this potting is an isolator [Does not conduct the current]

enter image description here

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  • \$\begingroup\$ If the potting material is an isolator, it could not be used as EMC shielding. \$\endgroup\$ – Uwe Sep 10 '19 at 10:53
  • \$\begingroup\$ Oh that is surprising ! Wasn't expecting anything like it ! But it makes kind of sense ! Maybe they realized after the initial design that their SMPS was hissing (or generating ultrasounds type noise). Thus the potting might in this case be a mean of reducing the switching noises. But why would they go to the trouble instead of finding another supply topology ? \$\endgroup\$ – benguru Sep 11 '19 at 7:29
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    \$\begingroup\$ Here is an application note on techniques to reduce SMPS noise. ti.com/lit/an/slua821/slua821.pdf \$\endgroup\$ – D Duck Sep 11 '19 at 7:43

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